Field of the Invention
The present invention relates to a quantum repeater network system, and more particularly, relates to a quantum repeater network system that employs repeaters to perform efficient quantum communication, regardless of a distance between one transmitter/receiver and the other transmitter/receiver on a network.
Description of the Related Art
Quantum communication technology is the foundation for, for example, quantum cryptography that guarantees the security based on principles of physics, and quantum teleportation used for transferring unknown states. The current quantum communication is performed based on direct transmission of optical pulses between one transmitter/receiver and the other transmitter/receiver. Therefore, as the optical loss in a communication channel between one transmitter/receiver and the other transmitter/receiver exponentially increases with the communication distance, communication resources that compensate for the optical loss would also increase exponentially with the communication distance. Specifically, for transmission of the same amount of information, the exponential increase of the optical loss causes the exponential increase of communication time or the exponential increase of the number of devices additionally provided to compensate for the optical loss. As a result, quantum communication performed based on the direct transmission of optical pulses has a limitation to communication enabled distances. For example, for quantum communication employing the present optical fiber transmission, a communication enabled distance is only about several hundreds of kilometers.
On the other hand, a quantum repeater system is well known as a method for extending a communication enabled distance by increasing the number of communication resources polynomially with the distance (see, for example, “H. J. Briegal et al., Phys. Rev. Lett. 81, 5932 (1998)” and “L. M. Duan, M. D. Lukin, J. I. Cirac and P. Zoller, Nature 414, 413 (2001)”). The quantum repeater system can supply one transmitter/receiver and the other transmitter/receiver with quantum entanglement by employing repeaters located between one transmitter/receiver and the transmitter/receiver.
FIG. 1 is a conceptual diagram illustrating a conventional quantum repeater system. As shown at step S11 in FIG. 1, transmission repeaters 103 and 104 and reception repeaters 105, 106 and 107 are alternately arranged between a transmitter/receiver 101 and a transmitter/receiver 102 that are located respectively for a sender and recipient. First, each of the transmission repeaters prepares optical pulses, each of which is in a bi-partite entangled state with a quantum system of the repeater, and then transmits the optical pulses to the adjacent reception repeaters. It should be noted that, at this time, the quantum systems of each repeater are in a separable state. The transmission repeater C103, for example, transmits to the reception repeater D1105 an optical pulse that is in an entangled state with quantum system c1, and transmits to the reception repeater D2106 an optical pulse that is in an entangled state with quantum system c2 (step S11). In this case, the quantum system c1 and the quantum system c2 are not in an entangled state, but are in a separable state.
On the other hand, each of the reception repeaters receives the optical pulses from the adjacent transmission repeaters or the transmitter of the adjacent transmitter/receiver, and the performs global measurement (e.g., Bell measurement) on the optical pulses (step S12) so as to make the quantum systems of the adjacent transmission systems in an entangled state (step S13). When the measurement succeeds, the process moves to the following step S13, and when the measurement fails, the process returns to step S11. For example, a reception repeater D1115 receives, from a transmission repeater C113, an optical pulse that is in an entangled state with the quantum system c1, and receives an optical pulse that is in the entangled state with a quantum system of a transmitter (sender) 111, and then performs global measurement (Bell measurement) for the received optical pulses (step S12).
Then, each of the transmission repeaters confirms that the quantum systems of the transmission repeater are in an entangled state with the quantum systems of the other transmission repeaters, and thereafter, performs global measurement (Bell measurement) for the quantum systems of the transmission repeater (step S13). For example, when both of reception repeaters D1115 and D2116 adjacent to the transmission repeater C123 have successfully performed the global measurement (step S12), since the quantum system c1 of the transmission repeater C123 and a quantum system of a transmitter (sender) 121 have quantum entanglement, and since the quantum system c2 of the transmitter repeater C123 and a quantum system of a transmission repeater 124 have quantum entanglement, the transmission repeater C123 performs global measurement on the remaining quantum systems c1 and c2 (step S13). The process of step S13 is sequentially performed by the transmission repeaters, and when all of the transmission repeaters have completed this process, quantum entanglement is supplied to a transmitter/receiver 131 and a transmitter/receiver 132 (step S14).
In order to provide such a quantum repeater system, typically, as described in the following reference documents, the transmission repeater necessitates matter qubits, such as the quantum system c1 and c2 in the above described example, that can interact with an optical pulse (request 1) and that can store quantum information in a long period of time (request 2; i.e., a memory function for quantum information). In addition to reference documents by “Briegel” and “Duan” previously described, there are other reference documents: “N. Sangouard et al., Rev. Mod. Phys. 83, 33 (2011)”, “L. Childress et al., Phys. Rev. Lett. 96, 070504 (2006)”, “P. van Loock et al., Phys. Rev. Lett. 96, 240501 (2006)”, “H. J. Kimbic, Nature 453, 1023 (2008)”, “K. Azuma et al., Phys. Rev. A 85, 062309 (2012)”, “M. Zwerger, W. Dür and H. J. Briegel, Phys. Rev. A 85, 062326 (2012)”, “Y. Li, S. D. Barrett, T. M. Stace, and S. C. Benjamin, arXiv:1008.1369”, and “W. J. Munro, R. V. Meter, S. G. R. Louis, and K. Nemoto, Phys. Rev. Lett. 101, 040502 (2008)”.
In another reference document, “W. J. Munro, A. M. Stephens, S. J. Devitt, K. A. Harrison, and K. Nemoto, Nature Photon. 6, 777 (2012)”, it is argued that so long as a very efficient single-photon source can be employed (request 3) and an efficient CZ gate between a matter qubit and single photon as request 1 described above is available, the quantum-memory function as in request 2 is not necessary. However, for all of the conventional quantum repeater systems, matter qubits with high functionality are indispensable components.